I have replaced the front heated oxygen sensor with a Mazda new unit to
fix a fault consistent with the P1135 indicator and I have cleared the
engine code/CEL. However, whenever I wind up the engine above 4000 RPMs,
the engine light, the P1135 code and 2 sensor indicators (whatever that
is) come up up again. I have searched the net for equivalent failure
modes and I can find some cases when this has occurred, but it appears
that people have not posted the subsequent solution. The engine does not
run rough at all at any RPM and there is no lost of engine power. The
P1135 code is the only one that gets triggered. This is a bit confusing,
since other failures associated with the coil, etc. triggered other
codes in some of the web postings I have read.

Is this an indicator of problems in the VICS, the coil pack, the
EGR/ducting or what? Unless the new Mazda OEM front O2 sensor is also
bad, I think the P1135 indicator is a mis-mapping of some other fault
cause.

P1135
H02S11 Heater Circuit Low (From my scanner Mazda code list)
DTC 1135 from 99 Miata Shop Manual
PCM voltage is below 5.8v when no power is supplied during 322-327 sec after
engine start to heated front O2 sensor.(From Shop Manual)
??? if no power is supplied, how can it have 5.8 v or any voltage for that
matter??

Possible causes
Wrong sensor heater resistance heater value or shorting wire to sensor?
I'd (on a 99) check the voltage between ECU(PCM) pin 1U and the ignition
switch and the sensor C pin
Also, grounding may be a problem.
The harness side of the connector pin layout is Long index to the top,
short index on bottom and each side.____
- A-C-
B-D
-

Work has interfered with diversions, so I have not devoted much time to
this lately. The Miata is my toy car, so it is aging nicely in the garage.

Before I got distracted, I did spray cleaned (with connector cleaner)
and reseted two harness connectors that I think are on the harness line
from the O2 sensor to some actuator that I think it may be related to
the VICS stuff. This is all from the back side of the cams cover to the
rear top side right side of the engine. I do not have schematics, so I
do not know what connects to what (computer, actuator, etc.). No
improvement. The connectors are well set and secured. I would venture to
guess that contact discontinuity due to vibration is not likely in this
2000 configuration. It is hot in the area behind the block, but I can
trigger the problem with the engine block cold if I rev. it up.

The problem does not occur at RPM lowers than 4500 or 5000. I could run
the car for miles at the lower RPMs without triggering the engine light
to go off.

My guess it that the O2 sensor error is a red herring, but I have no
idea what is signal flow in this engine management. So, I will break out
the electrical multimeter next to check wiring continuity and levels.
If any one knows of a good technical descriptions of engine management
for Mazda, it would be welcome.

Thanks for the replies so far. I know the answer will be trivial once it
is revealed.

First reply:
Many thanks to "xs11e" for the link to http://www.madracki.com/miata/ !!
It prints clearly despite the small fonts.
I assume that a text description of the operations related to diagram
B-1a (Engine Control System) on page Z-18 of file 001_2000wire.pdf would
be too much to ask in life? :-)
I suspect that a mode transition at high RPM (solenoids to VICS,
ignition coil 1 vs 2 use, etc.) may be at fault here. It may be that the
O2 sensor is (and was) actually working right, but the ignition process
saturates the sensor on those peak rev-up instants ??????
Thanks again.

Second reply:
The car is not likely to pass smog test since on the bi-annual test next
year the car has to go on the "thredmill". Any instantaneous reving-up
of the engine will set the engine light, even if the cause it not part
of the test cycle.

The light gets set by the event and it has to be cleared manually (via
the reader, BTN1 fuse removal or ground battery disconnect). I suspect
after the RPM go down, there is no instantaneous error condition, but I
do not know because the engine light does not auto-clear in my case. As
I understand it, the implementation of many engine light faults
auto-clears only after a large number (e.g., >40) of engine on-start-off
cycles with no faults.

I drove 80 miles yesterday not reving the engine past 4300 RPM or so. No
event. Today I instantaneously reved the engine to about 4800 RPM or so
and it triggered. However, I rather look at traffic on the freeway than
my tach when this happens, so I may not have the numbers exactly at the
maximum levels. It is very repeatable.

> GUEST wrote:
> I have replaced the front heated oxygen sensor with a Mazda
new
unit to
> fix a fault consistent with the P1135 indicator and I have
cleared
the
> engine code/CEL. However, whenever I wind up the engine above
4000
RPMs,
> the engine light, the P1135 code and 2 sensor indicators
(whatever
that
> is) come up up again. I have searched the net for equivalent
failure
> modes and I can find some cases when this has occurred, but it
appears
> that people have not posted the subsequent solution. The engine
does
not
> run rough at all at any RPM and there is no lost of engine power.
The

> P1135 code is the only one that gets triggered. This is a bit
confusing,
>
since other failures associated with the coil, etc. triggered other

> codes in
some of the web postings I have read.
>
> Is this an indicator of problems in
the VICS, the coil pack, the
> EGR/ducting or what? Unless the new Mazda OEM
front O2 sensor is
also
> bad, I think the P1135 indicator is a mis-mapping of
some other
fault
> cause.
>
> Thanks for any help or follow on discussion.

Rha: I am
currently having the same problem. Just purchased a
2000 Miata
and I got two codes last week, a PO300 and P1135. Fixed the plug
wiring
and no more PO300, but I can't keep the car from showing a P1135 once
I
go WOT. It appears to be the same problems you were having back in
'07. I am
curious on if you found a final diagnosis, was it a bad
ground? I've replaced
the plugs and plug wires. 'm tempted to just
take it in to the miata dealer but
will be getting my hands on a jack
next week so I think I'll wait to look at the
rear O2 sensor wiring.
Let me know if you have any input. Thanks for your time.

> My book shows operation at ~5300, not 4800 (I really don't know what the==> Problem happens at open throttle near 4500 RPM onwards. It is an RPM
and throttle combination, the best I can tell now after a few more runs.==> Air filter, plugs and plug cables are relatively new (<5000 miles).==> Fuel filter is original (12000 miles before recommended change), but
there is no hesitation at open throttle.

==> Can not measure it when the problem occurs, but it appears nominal
engine off and engine on at idle. Battery is relatively new and there is
no charge problem.

==> The new front O2 sensor heater resistances is nominal (in retrospect
inspection, so was the old). Voltage level to the heater element is
nominal. I have not gotten to the connectors to the PCM yet.

==> The car appears to be working normally otherwise, even at full
throttle. I am not sure that there is even an emission problem since it
is just an instantaneous fault that appears not to occur over 95%+ of
the driving time. The error code is still P1135. I have a while until I
have to inspect it again. I may have to take it to Mazda service so that
they can fix their own system and/or get it to pass inspection.

==> I do not have a good history with Mazda service departments. So I
was trying to avoid that experience again. It took many visits around
2001 to get them to fix a grinding and blocking 6-speed transmission on
this same car that I bought new. They tried to fix it the first time and
that was worst. Finally they replaced the gear box. They were very
reluctant to approve any warranty work. It may be that some harness
cable to the PCM got pinched due to these repairs and they finally
failed. Who knows. Mazda makes some interesting cars, but their service
has been mainly concerned with reducing cost to them rather than
maintaining customer satisfaction, at least in my prior warranty repair
case. However, once the customer is paying, the amount of work they are
willing to bill is tops. Other than for the transmission being bad from
the factory at the onset, I had no other non-maintenance repairs until
now (48,000 miles). My only regret is that I may have to deal with Mazda
service again. That is why I was trying at least to troubleshoot the
problem.

==> Thanks to all for the suggestions. When I get a final resolution, I
will post the outcome.

Well, I can feel your pain for having a problem. BUT, I can't feel the
the same pain that you have for a "dealer" that somehow people run into.
I live in Seattle WA. One of the dealers we have here is the "First"
dealer for Mazda in the US. I had a head gasket problem on my '91. It
was 1994 and way out of time and milage, but they replaced it. 4 years
(yes 4 years) later, it started to leak again. I took it in and after
they had the car for 2 days, ( a free loaner car to me ) they said that
they would relplace the gasket at no charge. Sooooooo, you see, it is
hard for me to think all dealer service is bad.

I hope you can find a dealer that will treat you well and fix your
problems.